Former President Nicolas Sarkozy’s hopes of a political comeback appeared to hang by a thread last night after evidence appeared to show he did try to subvert the French justice system in February.

Mr Sarkozy indignantly defended his “honour” earlier this month after investigating magistrates formally accused him of “active corruption” to derail other judicial investigations against him. He claimed in a television interview that the centre-left French government and “left-leaning” judges were conspiring to destroy his political career.

A leak to the newspaper Le Monde of the transcript of a bugged phone conversation has now potentially left the former President’s defence in tatters. In the transcript, reportedly tapped by investigating magistrates in February, Mr Sarkozy, says that he will “help” and “advance” the ambition of a senior French judge, Gilbert Azibert, who wished to obtain a post in Monaco. He also says that he will speak “to the Prince” and to the senior French official who runs the Monegasque government.

Other passages of Mr Sarkozy’s conversation with his lawyer, Thierry Herzog, suggest that Mr Azibert was providing inside information and influence with other judges for the former President inside France’s highest appeal court, the Cour de Cassation. In one passage, Mr Sarkozy’s lawyer, confirms Mr Azibert has “worked hard” for the ex-President. The Cour de Cassation was considering at the time legal rulings on different allegations of illegal campaign-financing and other alleged financial misconduct by Mr Sarkozy and associates.

The former President is alleged to have known that his telephones were being tapped. He was speaking on one of two mobiles obtained under a false name, “Paul Bizmuth”, by Mr Herzog. The laywer was speaking on the other “Bizmuth” phone.

A couple of days later, speaking on the phone registered under his name and then again on the Bizmuth phone, Mr Sarkozy changed tack. He told Mr Herzog he “didn’t feel right” about promoting Mr Azibert’s hopes. Mr Herzog agreed Mr Sarkozy “didn’t know [the judge] sufficiently”.

According to Le Monde, investigators believe that between the calls, Mr Sarkozy and Mr Herzog had been tipped off by persons unknown that the “Bizmuth” phones were also being tapped. The later conversations they believe, were staged to try to throw the investigation off the scent.

Mr Sarkozy never lobbied Monaco on Azibert’s behalf. The judge did not get the job. Under French law, however, it is illegal for a person to promise to use political influence to obtain a personal advantage.

The ex-President’s hopes of defeating the formal accusation of “active corruption” may now rest on a technicality. Mr Sarkozy’s legal team – backed by senior, independent laywers – argue the investigating magistrates acted illegally in tapping phone conversations between a lawyer and his client. If this prevails, the evidence may have to be dropped.

Meanwhile, another set of allegations against the former President and associates threatens to destroy France’s main opposition party. In the past week, senior figures in the centre-right Union pour un Mouvement Populaire (UMP) have been bombarding one another with leaks implying mis-spending of party funds.

The UMP is €74m (£58.8m) in debt, partly, it is alleged, because the party illegally paid for around €17m of hidden spending on Mr Sarkozy’s failed re-election campaign in 2012. Mr Sarkozy claims this “hidden spending” never happened. He suggests that the money was simply embezzled by other people in the party.